A. TECHNICAL FIELD
The present invention relates to an improvement of a primer composition for polyolefins.
B. BACKGROUND ART
Polyolefins such as polypropylene are relatively inexpensive and have excellent properties such as moldability and chemical resistance, so polyolefins are widely used in industrial fields including the automotive industry. Polyolefins are, for example, used as exterior parts for automobiles such as bumpers and molds, and surfaces thereof are usually coated to improve weather resistance as well as to afford effects of design.
Even if directly coated to polyolefin base materials, conventional paints are difficult to adhere to the polyolefin base materials. Thus, the adhesion to the base materials needs to be ensured by precoating primers to the base materials, and various primers are proposed. When the primers are used, surfaces of the polyolefin base materials are usually pre-washed with trichloroethane vapor. By this pre-washing, the surfaces of the polyolefin base materials are degreased, and partially dissolved into trichloroethane and thereby etched. In recent years, trichloroethane is cited as one of substances causing ozone layer destruction, and banned from being used. Instead, water base washing such as alkali washing, acid washing, or warm water washing is carried out. However, the water base washing has problems in that: the surfaces of the polyolefin base materials are degreased, but not etched, and therefore, even if primers are coated, the adhesion cannot sufficiently be ensured, and furthermore, the solvent resistance, especially, the gasohol resistance, of coating films is deteriorated, and the coating films peel off from parting portions thereof when high pressure washing of cars is carried out.
In order to solve these problems, polyolefin base materials to which polar substances such as low molecular polyesters or polyethers are added are used to improve the adhesion. However, the rise in the price of the base materials due to the addition of the polar substances is unavoidable.
Thus, it is hoped that a primer, which has sufficient adhesion to polyolefin base materials and excellent resistance to gasohol and high pressure washing of cars, will be developed.